Polysulfide rubbers are elastomeric solids which degrade on heating to elevated temperatures and also degrade when heated in the presence of traces of acid or base. These rubbers which are prepared by the reaction between alkali metal polysulfides and .alpha.,.omega.-dihalo organic compounds have the general backbone structure: EQU . . . RS.sub.x RS.sub.x R . . .
wherein R is an organic moiety derived by removal of the halogens from an .alpha.,.omega.-dihalo organic compound and x may range from about 1.8 to about 4.0, may have crosslinking introduced at random intervals through use of a small percentage of trihalide in the synthesis reaction, and may be either hydroxyl or thiol terminated. Polysulfide rubbers from the original synthesis are considered by the art to be hydroxyl terminated whereas those whose molecular weight has been reduced by the well-known process of splitting of some of the polysulfide links are considered by the art to be thiol terminated. Optimum properties of polysulfide rubbers are attained, as they are in natural rubber, by curing. The common curing agent for hydroxyl terminated polysulfide rubbers is zinc oxide. Oxidative cures are commonly used for thiol terminated polysulfide polymers, both the solid rubbers and the liquid polymers, although zinc oxide has been employed with these polymers also. The art has always considered that these cured rubbers were no different from the uncured rubbers in their lack of thermoplasticity especially in the presence of acids or bases. This lack of thermoplasticity of cured rubbers was always considered to be analogous to other cured or vulcanized rubbers which cannot be subjected to heat or pressure without undergoing a fundamental molecular degradation with loss of their original physical properties.
This is, of course, in contrast to the class of thermoplastic engineering resins as represented by poly(phenylenemono sulfide) which are not vulcanized and which can repeatedly be remelted and shaped without molecular alteration.
Strong base is known to degrade sulfide bonds forming various sulfide, sulfonate and sulfinate salts, and in the presence of oxygen, potassium hydroxide in particular is known to promote the oxidation of thiols to disulfides.
Solid polysulfide rubbers are known to have been prepared as latices in the presence of water soluble strong base such as sodium hydroxide, see U.S. Pat. No. 2,195,380 for example, but the subsequent water washing removes water soluble impurities, including any strong base present. Subsequent processing and cure removes any base not removed during the wash step so that essentially none of the base present during initial crude polymer formation is present in the final cured product.
The use of sodium or potassium hydroxide as deliquescent desiccating agents for use in "one package" cures of liquid polysulfide polymer based sealants is suggested in U.S. Pat. No. 3,225,017. There is no teaching or suggestion that the use of these particular deliquescent desiccants results in properties any different from the other compositions disclosed. U.S. Pat. No. 3,046,248 discloses the use of metal hydroxides including lithium hydroxide to cure liquid polysulfide polymers for making dental impressions. No teaching or suggestion that the presence of this particular alkali metal hydroxide affects thermoplasticity is contained in this patent.
The present invention is concerned with the unexpected finding that the incorporation of a catalytic amount of strong based into a cured solid polysulfide rubber induces softening on heating to a much greater degree than an untreated cured product followed by reversion to a rubbery solid possessing substantially the properties of the original on cooling.